RFC: Remove custom __deepcopy__ method from DimCoord

[jfrost-mo]

🚀 Pull Request

Description

Currently a request for comment, as there is probably a better way to fix this, and I've not done all the checklist stuff yet.

The custom copy implementation for DimCoords causes a RecursionError under python 3.14 as the super() proxy object is now copied, instead of its copy implementation being used on the original object.

This change does lose the read-only latching on copy that DimCoords currently have, which is what makes me think there is a better way. On the other hand, is the DimCoord not being created with read-only points coordinates?

Fixes #6762

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[jfrost-mo]

Unsurprisingly this broke the tests that were testing for it.

FAILED lib/iris/tests/unit/coords/test_DimCoord.py::Test_copy::test_writable_points - Failed: DID NOT RAISE <class 'ValueError'>
FAILED lib/iris/tests/unit/coords/test_DimCoord.py::Test_copy::test_realdata_readonly - Failed: DID NOT RAISE <class 'ValueError'>
==== 2 failed, 10420 passed, 54 skipped, 2403 warnings in 477.34s (0:07:57) ====